Cellular communications is quickly becoming an accepted and valuable facet of everyday life. At first, land-based cellular communication systems were used to provide coverage for the cellular systems. A typical terrestrial or land-based cellular communication system is illustrated in FIG. 1. FIG. 1 illustrates ten cells or regions C1-C10 in a typical land-based cellular mobile radio communication system. Normally a cellular mobile radio system would be implemented with more than ten cells. However, for the purpose of simplicity, the present invention can be explained using the simplified representation illustrated in FIG. 1. For each cell C1-C10, there is a base station B1-B10 with the same reference number as the corresponding cell. FIG. 1 illustrates the base stations as situated in the vicinity of the cell center and having omni-directional antennas. FIG. 1 also illustrates nine mobile stations M1-M9 which are moveable within a cell and form one cell to another. In a typical cellular radio communication system, there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purposes of explaining the present invention, the reduced number of mobile stations is sufficient.
Also illustrated in FIG. 1 is a mobile switching center MSC. The mobile switching center MSC illustrated in FIG. 1 is connected to all ten base stations B1-B10 by cables. The mobile switching center MSC is also co fixed switch telephone network or similar fixed network. All cables from the mobile switching center MSC to the base stations B1-B10 and the cables to the fixed network are not illustrated.
In addition to the mobile switching center MSC illustrated, there may be additional mobile switching centers connected by cables to base stations other than those illustrated in FIG. 1. Instead of cables, other means, for example, fixed radio links, may also be used to connect base stations to mobile switching centers. The mobile switching center MSC, the base stations and the mobile stations are all computer controlled. In land-based cellular communication systems, mobility functions, handover and location management where the network handles the movement of the users are developed and implemented in the mobile switching equipment, i.e., home location register, mobile switching center, base station controllers.
However, there is a need to provide coverage in areas where terrestrial coverage is not viable. Satellite systems may also be used to assure access over regions using different terrestrial standards. As a result, mobile communication via satellites is being developed as a complement to terrestrial mobile telephony. The satellites in these satellite communication systems are either in geostationary orbit, i.e., fixed orbit over a certain area of the earth, or non-geostationary orbits. A geostationary satellite mobile communication system is illustrated in FIG. 2. In this geostationary system, a satellite 10 is generally positioned 30,000 miles above the earth in a stationary position in relation to a point on the earth. A land base station 11 and a plurality of mobile stations can communicate with each other and other users around the world by transmitting and receiving signals to and from the satellite over feederlinks. Since the relative position of the satellite is fixed, geostationary communication systems do not have to consider handoff problems caused by the motion of a satellite. However, geostationary satellite communication systems have several drawbacks. Since the satellite is so far away from the earth, time delays on both the up and down feederlinks can create problems for two-way conversations. Furthermore, the limited output power and antenna gain/diversity of handheld mobile phones also limit the effectiveness of the geostationary satellite communication systems. In addition, huge antennas are needed to create the spot beams on the earth.
To achieve sufficient link margins and to support handheld mobile phones with limited output power and antenna gain/diversity, satellite systems using non-geostationary satellites are being considered. In such non-geostationary systems, the satellites move with respect to a point on the earth. As a result, handover and location management functionalities in the satellite communication network are needed in order to handle the motion of the network's satellites.
Associated with non-geostationary satellites, various techniques are known on how radio resources should be distributed on the ground via satellite beams. Several known techniques are Regionally Oriented Frequency Assignment (ROFA) and Satellite Oriented Frequency Assignment (SOFA). In ROFA, radio resources are assigned to regions meaning that the same geographical region always uses the same radio resource, i.e., frequency, timeslot, or spread spectrum code. Thus, in ROFA, the mobiles will see a fixed radio resource pattern, i.e., frequency reuse pattern, irrespective of the motion of the satellites. In SOFA, the beam from each satellite uses the same frequency irrespective of the satellite location. Thus, in SOFA, the radio resources are connected to the satellites. Thus, the mobiles will see the radio resource patterns, i.e., the frequency reuse pattern, move according to the motion of the satellites.
Due to the motion of the satellites in non-geostationary systems, mobility functions such as handover and location management to handle the satellite's motion need to be developed.